1. Field of the Invention
The invention relates to a molecular fluorine (F2) laser, and particularly to an F2-laser having enhanced efficiency, line-selection and line-narrowing of the selected line, and wavelength control.
2. Discussion of the Related Art
Semiconductor manufacturers are currently using deep ultraviolet (DUV) lithography tools based on KrF-excimer laser systems operating around 248 nm, and next generation ArF-excimer laser systems operating around 193 nm. Vacuum UV (VUV) lithography may use the F2-laser operating around 157 nm.
The emission of the F2-laser includes at least two characteristic lines around xcex1=157.629 nm and xcex2=157.523 nm. Each line has a natural linewidth of around 15 pm (0.015 nm). The intensity ratio between the two lines is |(xcex1)/|(xcex2)≈7. See V. N. Ishenko, S. A. Kochubel, and A. M. Razher, Sov. Journ. QE-16, 5 (1986). FIG. 1 illustrates the two above-described closely-spaced peaks of the F2-laser spontaneous emission spectrum.
Integrated circuit device technology has entered the sub-quarter regime, thus necessitating very fine photolithographic techniques. Line narrowing and tuning is required in KrF- and ArF-excimer laser systems due to the breadth of their natural emission spectra ( greater than 100 pm). Narrowing of the linewidth is achieved most commonly in these laser systems through the use of a wavelength selector consisting of one or more prisms and a diffraction grating (Littrow configuration). However, for an F2-laser operating at a wavelength of approximately 157 nm, use of a reflective diffraction grating may be unsatisfactory due to its low reflectivity and high oscillation threshold at this wavelength. In this regard, a master oscillator-power amplifier design has been proposed by two of the Applicants of the present application (see U.S. patent application Ser. No. 09/599,130, which is assigned to the same assignee as the present application and is hereby incorporated by reference) for improving the power of the output beam and enabling very narrow linewidths ( less than 1 pm), e.g., using a diffraction grating and or etalons, each preferably in combination with a beam expander. The tunability of the F2-laser has been demonstrated using a prism inside the laser resonator. See M. Kakehata, E. Hashimoto, F. Kannari, M. Obara, U. Keio Proc. of CLEO-90, 106 (1990). It is desired to provide a 157 nm beam from a narrow band F2 laser at a controlled wavelength.
F2-lasers are also characterized by relatively high intracavity losses, due to absorption and scattering in gases and all optical elements, particularly in oxygen and water vapor which absorb strongly around 157 nm. The short wavelength (157 nm) is responsible for the high absorption and scattering losses of the F2-laser, whereas the KrF-excimer laser operating at 248 nm, e.g., does not experience such losses. Therefore, the advisability of taking steps to optimize resonator efficiency is recognized herein.
It is therefore an object of the invention to provide a F2-laser wherein one of the plural emission lines around 157 nm is efficiently selected.
It is a further object of the invention to provide a F2-laser with efficient means for narrowing the selected line.
It is a further object of the invention to provide a F2-laser with line-selection and line-narrowing of the selected line, wherein the output wavelength is controlled.
In view of these and other objects, a F2-laser is provided including a discharge chamber filled with a gas mixture including molecular fluorine for generating a spectral emission including multiple closely spaced lines in a wavelength range between 157 nm and 158 nm including a primary line and a secondary line, multiple electrodes coupled with a power supply circuit for producing a pulsed discharge to energize the molecular fluorine, and a resonator including the discharge chamber, a transmissive interferometric device and a pair of resonator reflectors for generating a laser beam having a bandwidth of less than 1 pm. The interferometric device is configured for maximum transmissivity of a selected portion of the primary line and for relatively low transmissivity of the secondary line and an unselected portion of the primary line to substantially suppress the secondary line and the unselected portion of the primary line, thereby selecting and narrowing the primary line such that the F2-laser emits a single wavelength laser beam having a narrow spectral bandwidth that is less than the bandwidth of the primary line of a free-running F2-laser to provide a narrow band VUV laser beam.
A F2-laser is further provided including a discharge chamber filled with a gas mixture including molecular fluorine for generating a spectral emission including multiple closely spaced lines in a wavelength range between 157 nm and 158 nm including a primary line and a secondary line, multiple electrodes coupled with a power supply circuit for producing a pulsed discharge to energize the molecular fluorine, and a resonator including the discharge chamber, a reflective interferometric device, and another resonator reflector for generating a laser beam having a bandwidth of less than 1 pm. The interferometric device is configured for maximum reflectivity of a selected portion of the primary line and for relatively low reflectivity of the secondary line and an unselected portion of the primary line to substantially suppress the secondary line and the unselected portion of said primary line, thereby selecting and narrowing the primary line such that the F2-laser emits a single wavelength laser beam having a narrow spectral bandwidth that is less than the bandwidth of the primary line of a free-running F2-laser to provide a narrow band VUV laser beam.
A F2-laser is also provided including a discharge chamber filled with a gas mixture including molecular fluorine for generating a spectral emission including multiple closely spaced lines in a wavelength range between 157 nm and 158 nm including a primary line and a secondary line, multiple electrodes coupled with a power supply circuit for producing a pulsed discharge to energize the molecular fluorine, and a resonator including the discharge chamber, a transmissive interferometric device, and a pair of resonator reflectors for generating a laser beam having a bandwidth of less than 1 pm. The interferometric device is configured for maximum transmissivity of the primary line and for relatively low transmissivity of the secondary line to substantially suppress the secondary line, thereby selecting the primary line such that the F2-laser emits a single wavelength laser beam having a narrow spectral bandwidth that is less than the bandwidth of a free-running F2-laser to provide a narrow band VUV laser beam.
A F2-laser is further provided including a discharge chamber filled with a gas mixture including molecular fluorine for generating a spectral emission including multiple closely spaced lines in a wavelength range between 157 nm and 158 nm including a primary line and a secondary line, multiple electrodes coupled with a power supply circuit for producing a pulsed discharge to energize the molecular fluorine, and a resonator including the discharge chamber, a reflective interferometric device, and another resonator reflector for generating a laser beam having a bandwidth of less than 1 pm. The interferometric device is configured for maximum reflectivity of the primary line and for relatively low reflectivity of the secondary line to substantially suppress the secondary line, thereby selecting the primary line such that the F2-laser emits a single wavelength laser beam having a narrow spectral bandwidth that is less than the bandwidth of a free-running F2-laser to provide a narrow band VUV laser beam.
A F2-laser is also provided including a discharge chamber filled with a gas mixture including molecular fluorine for generating a spectral emission including multiple closely spaced lines in a wavelength range between 157 nm and 158 nm including a primary line and a secondary line, multiple electrodes coupled with a power supply circuit for producing a pulsed discharge to energize the molecular fluorine, and a resonator including the discharge chamber, a transmissive interferometric device, a dispersive optic and a pair of resonator reflectors for generating a laser beam having a bandwidth of less than 1 pm. The dispersive optic is arranged at a particular orientation for dispersing the multiple closely-spaced lines including the primary and secondary lines such that only the primary line remains within an acceptance angle of the resonator and any other line(s) including the secondary line are dispersed outside of the acceptance angle of the resonator. The interferometric device is configured for maximum transmissivity of a selected portion of the primary line and for relatively low transmissivity of an unselected portion of the primary line to substantially suppress the unselected portion of said primary line. The dispersive optic and interferometric device thereby select and narrow the primary line such that the F2-laser emits a single wavelength laser beam having a narrow spectral bandwidth that is less than the bandwidth of the primary line of a free-running F2-laser to provide a narrow band VUV laser beam.
A F2-laser is also provided including a discharge chamber filled with a gas mixture including molecular fluorine for generating a spectral emission including multiple closely spaced lines in a wavelength range between 157 nm and 158 nm including a primary line and a secondary line, multiple electrodes coupled with a power supply circuit for producing a pulsed discharge to energize the molecular fluorine, and a resonator including the discharge chamber, a reflective interferometric device, a dispersive optic and another resonator reflector for generating a laser beam having a bandwidth of less than 1 pm. The dispersive optic is arranged at a particular orientation for dispersing multiple closely-spaced lines including the primary and secondary lines such that only the primary line remains within an acceptance angle of the resonator any other line(s) including the secondary line are dispersed outside of the acceptance angle of the resonator. The interferometric device is configured for maximum reflectivity of a selected portion of the primary line and for relatively low reflectivity of an unselected portion of the primary line to substantially suppress the unselected portion of the primary line. The dispersive optic and interferometric device thereby select and narrow the primary line such that the F2-laser emits a single wavelength laser beam having a narrow spectral bandwidth that is less than the bandwidth of the primary line of a free-running F2-laser to provide a narrow band VUV laser beam.
A F2-laser is also provided including a discharge chamber filled with a gas mixture including molecular fluorine for generating a spectral emission including multiple closely spaced lines in a wavelength range between 157 nm and 158 nm including a primary line and a secondary line, multiple electrodes coupled with a power supply circuit for producing a pulsed discharge to energize the molecular fluorine, and a resonator including the discharge chamber and an interferometric device for generating a laser beam having a bandwidth of less than 1 pm. The interferometric device is configured for relatively suppressing the secondary line and a portion of the primary line other than a selected portion of the primary line to substantially suppress the secondary line and the unselected portion of the primary line compared with the selected portion of the primary line, thereby selecting and narrowing the primary line such that the F2-laser emits a single wavelength laser beam having a narrow spectral bandwidth that is less than the bandwidth of the primary line of a free-running F2-laser to provide a narrow band VUV laser beam.
A F2-laser is further provided including a discharge chamber filled with a gas mixture including molecular fluorine for generating a spectral emission including multiple closely spaced lines in a wavelength range between 157 nm and 158 nm including a primary line and a secondary line, multiple electrodes coupled with a power supply circuit for producing a pulsed discharge to energize the molecular fluorine, and a resonator including the discharge chamber and an interferometric device for generating a laser beam having a bandwidth of less than 1 pm. The interferometric device is configured for relatively suppressing the secondary line to substantially suppress the secondary line compared with the primary line, thereby selecting the primary line such that the F2-laser emits a single wavelength laser beam having a narrow spectral bandwidth that is less than the bandwidth of a free-running F2-laser to provide a narrow band VUV laser beam.
A F2-laser is further provided including a discharge chamber filled with a gas mixture including molecular fluorine for generating a spectral emission including multiple closely spaced lines in a wavelength range between 157 nm and 158 nm including a primary line and a secondary line, multiple electrodes coupled with a power supply circuit for producing a pulsed discharge to energize the molecular fluorine, and a resonator including the discharge chamber, an interferometric device and a dispersive optic for generating a laser beam having a bandwidth of less than 1 pm. The dispersive optic is arranged at a particular orientation for dispersing the multiple closely-spaced lines including the primary and secondary lines such that only the primary line remains within an acceptance angle of the resonator any other line(s) including the secondary line are dispersed outside of the acceptance angle of the resonator. The interferometric device is configured for relatively suppressing an unselected portion of the primary line to substantially suppress the unselected portion of the primary line. The dispersive optic and interferometric device thereby select and narrow the primary line such that the F2-laser emits a single wavelength laser beam having a narrow spectral bandwidth that is less than the bandwidth of the primary line of a free-running F2-laser to provide a narrow band VUV laser beam.
A F2-laser is further provided including a discharge chamber filled with a gas mixture including molecular fluorine for generating a spectral emission including multiple closely spaced lines in a wavelength range between 157 nm and 158 nm including a primary line and a secondary line, multiple electrodes coupled with a power supply circuit for producing a pulsed discharge to energize said molecular fluorine, and a resonator including the discharge chamber and an interferometric device for generating a laser beam having a bandwidth of less than 1 pm. The interferometric device is configured for relatively suppressing the secondary line and an unselected portion of the primary line to substantially suppress the secondary line and the unselected portion of the primary line compared with a selected portion of the primary line, thereby selecting and narrowing the primary line such that the F2-laser emits a single wavelength laser beam having a narrow spectral bandwidth that is less than the bandwidth of the primary line of a free-running F2-laser to provide a narrow band VUV laser beam. A wavelength monitor is coupled in a feedback loop with a processor for monitoring a spectral distribution of the laser beam. The processor controls an interferometric spectrum of interferometric device based on the monitored spectral distribution such that sidebands within the spectral distribution are substantially minimized.
A F2-laser is also provided including a discharge chamber filled with a gas mixture including molecular fluorine for generating a spectral emission including multiple closely spaced lines in a wavelength range between 157 nm and 158 nm including a primary line and a secondary line, multiple electrodes coupled with a power supply circuit for producing a pulsed discharge to energize the molecular fluorine, and a resonator including the discharge chamber and an interferometric device for generating a laser beam having a bandwidth of less than 1 pm. The interferometric device is configured for relatively suppressing the secondary line to substantially suppress the secondary line compared with the primary line, thereby selecting the primary line such that the F2-laser emits a single wavelength laser beam having a narrow spectral bandwidth that is less than the bandwidth of a free-running F2-laser to provide a narrow band VUV laser beam. The wavelength monitor is coupled in a feedback loop with a processor for monitoring a spectral distribution of the laser beam. The processor controls an interferometric spectrum of the interferometric device based on the monitored spectral distribution such that sidebands within the spectral distribution are substantially minimized.
A F2-laser is further provided including a discharge chamber filled with a gas mixture including molecular fluorine for generating a spectral emission including multiple closely spaced lines in a wavelength range between 157 nm and 158 nm including a primary line and a secondary line, multiple electrodes coupled with a power supply circuit for producing a pulsed discharge to energize the molecular fluorine, and a resonator including the discharge chamber, an interferometric device and a dispersive optic for generating a laser beam having a bandwidth of less than 1 pm. The dispersive optic is arranged at a particular orientation for dispersing the multiple closely-spaced lines including the primary and secondary lines such that only the primary line remains within an acceptance angle of the resonator and any other line(s) including the secondary line are dispersed outside of the acceptance angle of the resonator. The interferometric device is configured for relatively suppressing an unselected portion of the primary line to substantially suppress the unselected portion of the primary line. The dispersive optic and interferometric device thereby select and narrow the primary line such that the F2-laser emits a single wavelength laser beam having a narrow spectral bandwidth that is less than the bandwidth of the primary line of a free-running F2-laser to provide a narrow band VUV laser beam. A wavelength monitor is coupled in a feedback loop with a processor for monitoring a spectral distribution of the laser beam. The processor controls an interferometric spectrum of the interferometric device based on the monitored spectral distribution such that sidebands within the spectral distribution are substantially minimized.